ANEW PREPARATION TECHNIQUE FOR CALCAREOUS NANNOFOSSILSFROM ORGANIC-RICH ARGILLACEOUS SEDIMENTS
by Magdy H. Girgis
Robertson Research International Ltd., Llandudno, Gwynedd, North Wales, U.K.

INTRODUCTION
The technique described below was developed to extract nannofossils from theSenonian, organic-rich, micaceous, often phosphatic, silty shale ("HotShales") sequences in well sections drilled offshore West Africa.
Thetechnique may be applied to similar sediments of different ages withappropriate modifications.
The "Hot Shales" either contain very etched assemblages of calcareous
nannofossils, dominated by the solution-resistent species MiauZa staurophora,or are occasionally barren.
The assemblages recovered by conventionaltechniques (smear and short centrifuge as described by Taylor and Hamilton,
1982) contain moderately abundant specimens and show very low species
diversity.
Age de terminations based on these nannofossil assemblages andother microfossils (foraminifera and palynomorphs) are essentially broad.
This has prompted research into increasing the nannofossil concentration inorder to observe more taxa and thus provide more detailed age determinations.
These aims were achieved mainly by removing large proportions of thenon-nannofossil particles, firstly by sieving (to remove particles >10 ~m),secondly by centrifuging (to remove particles <3 ~m), and thirdly by
floatation (to remove kerogen) and then settling (to remove heavy minerals).
As nannofossil concentration increases through these stages, a slide may be
prepared after each stage until an age diagnostic assemblage is obtained,
thus avoiding unnecessary treatments.
DISCUSSION
The organic matter in sediments is either soluble in organic solvent (oil andbitumen) or insoluble (kerogen, sensu stricto).
Removing soluble organic
matter from apparently barren Upper Liassic bituminous rocks has been proved
to liberate nannofossils, resulting in rich assemblages being obtained(Herold-Vieuxble, 1979). The soluble fraction (oil) in the present material
is negligible (less than 0.5% by weight) and the removal of this does notappear to increase the abundance of nannofossils, although it produces
cleaner ~pecimens. The oil was extracted by a mixture of dichloromethane andmethanol (95:5 by volume) in Soxhlet apparatus, which was run for sevenhours. This extraction was described in detail by Herold-Vieuxble (1979).
The present technique was carried out on oil-free samples.
Kerogen and heavy minerals are known to have respectively lower and higher
densities than calcite (specific gravity 2. 715), of which nannofossils arecomposed, and thus a heavy liquid is needed to separate them. A mixture oftetrabromoethane (TBE) and acetone is the liquid chosen because it is aneutral solution (pH = approximately 7), easy to handle and can be preparedto give the wide range of densities necessary to separate both kerogen andheavy minerals. The density of the liquid varies between approximately 0.8
and 3. 0 depending on the proportions of TBE and ace tone used, as shown inFig. 1. In order to determine the optimum densities of the liquid at which93
INA Newsletter vol.8- 1986

kerogen and heavy minerals could be separated from nannofossils, theabundance of nannofossils (number of specimens in 100 fields of view) wasrecorded in both the float and sink fractions in the liquid at different
densities.
The relative abundances (in percentages) were calculated andpresented in Fig. 2. Almost all the specimens (mainly M. staurophora) floatat specific gravity 2.2 and sink at 2.9.
100~--------------------------------------------------------r

25

75§"FIG. 1. Diagram
showtng the range of
specific gravity oftetrabromoethane I
acetone at 20°C.

g
§"g.
.
c0

50

50..
«!
i....
~..

75

25

0.79

1.5

1.0

2.0lOO
2.98

2.5

Spoclfic gravity

lOO

90

1080

2070
FIG. 2 Diagram showing
relattve abundances
ltn percentages! ofnannofossils in the floatand sink fractions atdtfferent denstties.

~

:§30

FLOAT

60

4050~

..

"'0~ 50
z00

c4060

SINK
30

70

2080

1090:;

..
zlOO

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

Specific gravity

METHODS(1) Sieving: about 2cm 3 of oil-free sample was disintegrated in distilledwater overnight, placed in ultrasonic vibration for a few seconds,
shaken well and then passed through a 10 ~m nylon sieve. Five to ten
minutes are usually required to collect enough filtrate. A narrow jetof distilled water may be used to accelerate the process and ensure
efficient sieving. The presence of a few 11-13 ~m particles was notedin the filtrate, due to the deviation of the sieve's pores from circularshape and/or to pressure applied on the sieve. Considerable quantities
of kerogen were removed by this sieving process.

94

INA Newsletter vol.B- 1986

(2)Centrifuging: the filtrate (<10 ~m fraction) was centrifuged at1000 r.p.m. for 104 seconds and decelerated using a brake.
The
supernatant containing particles <3 ~m was discarded and the precipitate
retained.
The process was repeated on the precipitate until thesupernatant was nearly clear. The speed and time are calculated for an
Heraeus Christ model Labofuge 6000.
The calculations to derive therequired times and speeds for particle sizing are outlined in Katz
(1978).

(3)

Floatation: the dry residue from stage (2) was suspended in TBE/acetoneof 2.2 sp. gr. and centrifuged at 3000 r.p.m. for 10 minutes toaccelerate the separation.
The floating kerogen and most of theTBE/ acetone was pipetted off. The residue was resuspended in acetone
and then centrifuged at 3000 r.p.m. for 10 minutes to settle thesuspension. Acetone was then pipetted off and the residue was dried.

(4)

Settling: the dry residue from (3) was suspended in TBE/acetone of
2. 9 sp. gr. and centrifuged at high speed as in (3) to accelerate theseparation.
The float containing nannofossils was collected by apipette and sufficient acetone was added to lower the specific gravity
to at least 2.0. This mixture was shaken well, centrifuged at highspeed and the supernatant was decanted off. This treatment on the float
fraction was repeated three times to remove the TBE completely. The
supernatant in the last stage was pipetted off, and the residue wasdried and resuspended in distilled water ready for mounting on slides.

RESULTS
The abundance and species diversity of the nannofloral assemblages recoveredby the present technique are considerably greater than that achieved byconventional preparations. The concentration is 4 to 6 times greater anddiversity is 1.5 to 5 times higher. Among the age-restricted indicators
which were observed only after the present technique had been employed are
Quadx>um trifidwn, Broinsonia paraa aonstriata and many specimens of theArkhangeZskieZZa aymbifo~is group, sufficient for the determination of thegroup's mean length.
This mean length was found to be important
stratigraphically (Girgis, 1985).
ACKNOWLEDGEMENT
Thanks to the Directors of Robertson Research International Ltd. for thepermission to publish this paper, Mr. K. West (RR Laboratory) for technicalassistance and stimulating discussions, and Dr. P.R. Marshal! (RR PetroleumServices) for critically reading the manuscript.
REFERENCES
GIRGIS, M.H.
(1985): Upper Cretaceous
Lower Tertiary calcareous
nannofossils :from Egypt and Spain. Unpublished Ph.D. thesis, London
University, 491 p.
HEROLD-VIEUXBLE, I. (1979): A new preparation technique for calcareous
nannoplankton from bituminous rocks.
Micropaleont., vol. 25, no. 4,pp. 438-439.
KATZ, B.J. (1978): Preparation of calcareous nannofossil assemblages forchemical examination. Jour. Paleont., vol. 52, no. 2, pp. 497-500.
TAYLOR, R.J. AND HAMILTON, G.·B. (1982): Techniques. In Lord, A.R. (ed.); A
Stratigraphical Index of Calcareous Nannofossils; Ellis Horwood,Chichester, pp. 11-15.

INA Newsletter vol.S- 1986

95